rasterizer: fix warning
rasterizer: fix warning

While int8_t work in practice, uint8_t is more correct type here.

ShiftMediaProject/libass,Akemi/libass,libass/libass,MrSmile/libass,Akemi/libass,ShiftMediaProject/libass,astiob/libass,libass/libass,xbmc/libass,astiob/libass,MrSmile/libass,Akemi/libass,xbmc/libass